libstdc++: Fix noexcept guarantees for ranges::split_view

libstdc++: Fix noexcept guarantees for ranges::split_view

Also introduce the _M_i_current() accessors to solve the problem of
access to the private member of _OuterIter from the iter_move and
iter_swap overloads (which are only friends of _InnerIter not
_OuterIter).

	* include/std/ranges (transform_view::_Iterator::__iter_move): Remove.
	(transform_view::_Iterator::operator*): Add noexcept-specifier.
	(transform_view::_Iterator::iter_move): Inline __iter_move body here.
	(split_view::_OuterIter::__current): Add noexcept.
	(split_view::_InnerIter::__iter_swap): Remove.
	(split_view::_InnerIter::__iter_move): Remove.
	(split_view::_InnerIter::_M_i_current): New accessors.
	(split_view::_InnerIter::__at_end): Use _M_i_current().
	(split_view::_InnerIter::operator*): Likewise.
	(split_view::_InnerIter::operator++): Likewise.
	(iter_move(const _InnerIter&)): Likewise.
	(iter_swap(const _InnerIter&, const _InnerIter&)): Likewise.
	* testsuite/std/ranges/adaptors/split.cc: Check noexcept-specifier
	for iter_move and iter_swap on split_view's inner iterator.